Crystal structure of 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene

The title complex is composed of saddle-shaped molecules which closely interact in a pairwise fashion through π–π and C—H⋯π contacts to form ‘dimers’. These ‘dimers’ further interact through C—H⋯S and C—H⋯π contacts to construct a complex three-dimensional extended structure.


Chemical context
Since the first report on 9,10-bis(1,3-dithiol-2-ylidene)-9,10dihydroanthracene (exTTF) (I) as a highly-conjugated electron donor (Bryce & Moore, 1988), numerous studies have been conducted on the development of exTTF derivatives that are applicable toward organic electronics. (Brunetti et al., 2012) To our surprise, the single crystal structure of exTTF has not been reported and most of the existing literature on exTTF focuses on theoretical calculations and modeling. (Gruhn et al., 2006;Zhao & Truhlar, 2008) Herein, we report the single-crystal structure of exTTF.
The average C-C bond length within the benzene rings (excluding the edges shared with the central ring) is 1.391 Å as is typical of phenyl rings. The length of the edges shared with the central ring are slightly longer C5-C10 = 1.419 (2) Å and C12-C17 = 1.412 (2) Å . The remaining C-C distances making up the central ring are longer still with an average of 1.477 Å . Since the distances within the central ring are in between those of typical C-C single and double bonds; this supports the idea of a highly delocalized bonding motif throughout the dihydroanthracene ring system. The bond distances between the dihydroanthracene and the 1,3-dithiol-2-ylidene groups are on the order of typical C C bonds, C3 C4 = 1.360 (2) Å and C11 C18 = 1.361 (2) Å .

Supramolecular features
Through a series of C-HÁ Á Á andinteractions, each molecule of (I) closely interacts with a neighboring molecule to form a 'dimer', Fig. 2. Theinteraction is between the C1-C2-S2-C3-S1 ring and the C1 i -C2 i -S2 i -C3 i -S1 i ring [symmetry operation: (i) Àx + 1, Ày + 1, Àz + 1] and is rather long at 4.068 (15) Å . There are five C-HÁ Á Á interactions between the two molecules in which atoms H1 and H2 of one molecule interact with various systems of the neighbor. The shortest contact is between H1 and the C11 i C18 i double bond at 2.606 (12)     A view of a 'dimer' of (I) showing (a) how the 1,3-dithiol-2-ylidene group of one molecule sits in the U-shape of a neighboring molecule, and (b) the interactions between the molecules that make up the 'dimer'. Gray = Carbon, yellow = Sulfur, green = Hydrogen, blue dashed line =interaction, red dashed line = C-HÁ Á Á interaction. [Symmetry operation: (i) Àx + 1, Ày + 1, Àz + 1.]

Figure 3
A portion of the two-dimensional network formed between 'dimers' when only the dihydroanthracene CH interactions are taken into account viewed (a) along the a axis, and (b) along the b axis. Blue dashed lines are intra-dimer interactions and red dashed lines are dihydroanthracene CH inter-dimer interactions.

Database survey
Many derivatives of (I) have been crystallographically characterized with various substituents on the dihydroanthracene, the dithiol, or both moieties. A search of the Cambridge Crystal Database (CCD) (Groom & Allen, 2014) yields three derivatives of (I) with substituents on the dihydroanthracene and twelve derivatives with substituents on both the dihydroanthracene and the dithiol. There have been twentynine structures reported in the CCD with substituents on the dithiol ring. The complex most closely related to (I) is the tetramethyl-substituted 9,10-anthracenediylidene-2,2 0 -bis(4,5dimethyl-1,3-dithiole) (Bryce et al., 1990;CCD code: JIJGIS). This molecule crystallizes in the same space group as (I) (monoclinic, P2 1 /c) and has a similar saddle shape. It also appears to form similar 'dimers' in which there are both C-HÁ Á Á andinteractions between the two molecules.
A recent computational study focused on predicting the most energetically favored 'dimers' of (I) (Denis & Iribarne, 2015). This study predicted the 'dimer' characterized in (I) as the second most favorable, being 1.7 kcal mol À1 less stable than the predicted favorite. The study detailsstacking between two of the dithiol rings, C-HÁ Á Á contacts between the dithiol H atoms and the anthracene rings,stacking between anthracene units, as well as an interaction between the partial positive charge of the S atoms and the anthracene rings for the preferred computational 'dimer'. The study briefly describes the C-HÁ Á Á andinteractions found in (I), but states that the lack ofstacking between the anthracene moieties is the reason this orientation is slightly less favorable.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. A structural model consisting of the target molecule was developed. H atoms were included as riding idealized contributors with C-H = 0.95 Å U iso (H) = 1.2U eq (C).

Special details
Experimental. One distinct cell was identified using APEX2 (Bruker, 2014). Fourteen frame series were integrated and filtered for statistical outliers using SAINT (Bruker, 2014) then corrected for absorption by integration using SAINT/SADABS v2014/2 (Bruker, 2014) to sort, merge, and scale the combined data. No decay correction was applied. Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Structure was phased by direct methods (Sheldrick, 2015). Systematic conditions suggested the unambiguous space group. The space group choice was confirmed by successful convergence of the full-matrix leastsquares refinement on F 2 . The final map had no significant features. A final analysis of variance between observed and calculated structure factors showed little dependence on amplitude and resolution.